In the past more than 20 years burners with short but effective premixing zones (so-called EV burners: environmental friendly V-shaped burners) have been implemented in several gas turbines of the applicant, with very low NOx levels. In addition to this, three variants of premix technologies have been successfully developed and deployed into those gas turbine engines: the sequential EV burners —a technology that allows premixing of natural gas and oil into a hot exhaust stream to reheat the exhaust gases of a first high pressure turbine; the MBtu EV burners that are used to burn syngas in a premix flame with low NOx emissions; and the advanced EV burners (AEV) that are capable to prevaporize and premix liquid fuel prior to combustion and burn it with very low NOx emissions without water injection.
Document EP 0 851 172 A2 discloses an exemplary EV burner of the double-cone type, for operating a combustion chamber with a liquid and/or gaseous fuel, whereby the combustion air required for this purpose is directed through tangential air-inlet ducts into an interior space of the burner. This directing of the flow results in a swirl flow in the interior space, which swirl flow induces a backflow zone at the outlet of the burner. In order to stabilize the flame front forming there, at least one zone is provided at each sectional body forming the burner, within which zone inlet openings are provided for the injection of supplementary air into the swirl flow. Due to this injection, a film forms at the inner wall of the sectional bodies, which film prevents the flame from being able to flashback along the inner wall of the sectional bodies into the interior space of the burner.
Document EP 2 423 597 A2 shows another exemplary EV burner in the form of a double-cone burner, which has two partial cone shells which are arranged nested one inside the other, forming air inlet ducts between them, through which combustion air from the outside flows into a conical inner space of the premix burner. Linear rows of holes of injection openings, which extend transversely to the flow direction of the combustion air, are arranged on the outer walls of the air inlet ducts and through which a gaseous fuel is injected into the combustion air which flows past transversely to them.
Document DE 195 45 310 A1 discloses a further pre-mixing burner consisting of a hollow cone with an outer and inner cone casing. At least two inlet ducts run at a tangent to the inner cone casing and are positioned along a straight cone casing line. The part cone axes of the part shells formed lie on the same cone axis. The pre-mixing burner is divided into at least two, for example four, parts containing the inlet ducts so as to swirl the combustion air. A fuel nozzle is positioned at the cone tip for injecting liquid fuel.
Document EP 0 704 657 A2 describes a combustor arrangement, which is shown in FIG. 1. The combustor arrangement comprises a premix burner 11 with a swirler 15 and a mixing tube 14. The swirler 15 has a conical structure which is repeatedly acted upon by a combustion-air flow 19 entering tangentially. The flow forming herein, with the aid of a transition geometry provided downstream of the swirler 15, is passed smoothly into a transition piece 16 in such a way that no separation regions can occur there. The transition piece 16 is extended on the outflow side of the transition geometry by the mixing tube 14, the transition piece 16 and the tube 14 forming the actual mixing zone of the burner. The transition piece 16 and the mixing tube 14 are connected by a ring 18. Furthermore, transition passages 201 bridge the difference in cross-section without at the same time adversely affecting the flow formed.
A combustion chamber 12 adjoins the end of the mixing tube 14, there being a jump in cross-section between the two cross-sections of flow. Only here does a central backflow zone form, which has the properties of a flame retention baffle. The combustion chamber 12 has a front panel 13 with an opening for receiving the end of the mixing tube 14.
As shown in FIG. 1, in principle, to lower NOx formation, the fuel air mixing can be extended in a mixing tube flow 20 by applying such a mixing tube 14 at the exit of a swirler 15 prior to the sudden area expansion entering the combustion chamber 12 where the flame is stabilized by the recirculating flow. The air slots and gas channels of the swirlers 15 are confined in the downstream part by an intersecting plane orthogonal to the burner axis. Therefore the swirler exit is not circular and needs the special transition piece 16 in-between the swirler 15 and the cylindrical mixing tube 14. This transition piece is prone to flash-back as it must apply a complicated 3D shape to avoid flow separation and flame stabilization in recirculation zones.
The gas turbine burner is usually mounted to the combustion chamber such that it can move in axial direction to compensate for thermal expansion. Leakages through the seals applied between the burner and the front panel of the combustion chamber are close to the flame and cause disturbances to the oxidation process. An additional problem is caused by the approach flow to the burner which is subject to flow disturbances due to the narrow space within the combustor hood.
Thus, the current state-of-the-art distinguishes between the front panel 13 of the combustion chamber 12 and the burner 11. To minimize air leakages, a seal is applied, which however is not tight, since the burner 11 and combustion chamber 12 are subject to large temperature changes and the burner 11 must be axially movable with regard to the front panel 13. Since this seal is close to the flame at the exit of the burner the leakage air is disturbing the oxidation process of the flame by local flame quenching effects. To even out the approach flow, currently, a sieve is shrouded around the burner, however, causing a pressure drop to the air flow.